The present invention relates to an improved removable system for clamping an object, such as a heater or a sensor or both, to a cylindrical member, such as a nozzle or hot runner channel, in injection molding equipment and to a method for applying a force for clamping an object to a cylindrical member.
Heat generation and management of the molten material represents one of the most critical aspects in an injection molding process. Heating of the molten material that travels through the mold manifold and hot runner nozzles is accomplished using various phenomena, techniques and components. The methods and the devices for heating the molten material have to meet stringent design and process constraints that are specific to the injection molding process, to the molten material to be processed, and to the type of article to be molded.
Electrical heating represents the most popular heating method and various electrical heaters have been developed for manifolds and hot runner nozzles. In this regard, coil heaters, band heaters, and cartridge heaters, that can be sometimes embedded or integrated in the nozzle housing, can be mentioned. Integrated electrical heaters are very expensive, very difficult to manufacture, and impossible to replace, unless one sacrifices the entire nozzle. In many instances, it is preferable to use removable electrical heaters that are less expensive, can be easier manufactured, assembled, tested, and serviced. One major problem that has not been solved satisfactory so far is related to the clamping of the heater to the element to be heated so that an intimate thermal contact is established with minimal heat loss.
Several clamping techniques have already been developed but there is still a need for improved removable clamping methods and devices so that the heater is more compact, more easily attached and detached, made of simpler and more reliable components that do not lose their properties in time, and provide an enhanced thermal contact with the element to be heated.
Generally, there are four main design concepts or types used to removably attach a heater to the manifold or nozzle housing of a hot runner nozzle. For example, there are so called "clampless" heaters which do not have a clamp mechanism per se, but instead rely on an extremely precise fit between the heater and the nozzle, thus requiring no extra clearance other than for the outer diameter of the heater. Aside from the high cost of manufacturing both fitting diameters, there are additional drawbacks. For example, it can become necessary to engineer additional devices to trap the heater on the nozzle to prevent it from slipping axially away from its installed position during handling or movement of the mold. These heaters also tend to have a thick wall section, on the order of 4 mm for heaters with a 12-42 mm internal diameter. Also, should any burrs or surface imperfections exist on the mating surfaces, the heater can seize on the nozzle and become very difficult to remove without damage to the heater or the nozzle.
The mechanical clamping techniques use fastening means and specialized tools to attach and detach the electrical heater. To obtain effective and efficient heat transfer from the heater to the heated part, it is necessary to have close contact between them. This is generally accomplished by use of a clamping device such as a metal shroud which encompasses the heater body and is screwed together at its ends to draw the heater tighter against the nozzle to be heated. This method has an advantage of being simple, using common tools such as a screw driver to tighten or loosen the heater. However, in cases where the heater must be installed in a confined area, it can be difficult to access the screw head for tightening purposes once it is in its correct orientation in the molding machine. Typically, the obstruction is the mold plate adjacent to and surrounding the heater. In such cases, additional clearance is often machined into the plate to permit the tightening tool to reach the screw head. In situations where it is prohibitive to add clearance machining because it will compromise the strength of the mold plate or reduce the plate material available to back-up and support other mold components, a design such as that shown in U.S. Pat. No. 4,968,247 to Olson and U.S. Pat. No. 5,263,230 to Johnson may be used to permit tightening of the heater by way of a cam actuated clamp housing. This type of design permits a tool to approach from the axial direction of the heater, thus requiring no special clearance for the tightening tool. While this is an improvement for ease of assembly and structural integrity of the mold plate, there is still the need to cut a small pocket of clearance for the cam mechanism which stands outside of the cylindrical profile of the outer surface of the heater. Likewise, there is a variety of other tightening devices available for use on the market, which also invariably add substantially to the outer diameter of the heater. Further reference is made to U.S. Pat. No. 4,268,241 to Rees, which shows another way to clamp the heater using a threaded sleeve.
The so called "flexible" clamping methods use variable deformable, spring-like or elastic elements to apply a clamping force upon the heater and keep it in intimate thermal contact with the melt channel of the manifold or nozzle housing. Reference is made in this regard to U.S. Pat. No. 5,411,392 to Van Buren. The Van Buren '392 clamp is successfully applicable to band or sheet heaters having spring like characteristics. The slot in the heater that provides the expansion feature also reduces the heat on a certain portion of the nozzle. Van Buren '392 however is not recommended to clamp coil heaters. Other U.S. Patents, such as Rees '241 and U.S. Pat. No. 5,113,576 to van Boekel, show embodiments of electrical heater clamping elements where the mechanical elastic deformation created by a "wedge effect" at the interface between the heater and the nozzle is used to generate a less secure clamping force.
In the so called "thermal" clamping methods, the clamping force is generated by the different thermal expansion coefficients between the elements forming the heating system and/or the nozzle housing. Thermal clamping methods in most cases are implemented by using bimetallic elements in the construction of the heater. Reference is made in this regard to U.K. Patent No. 1,290,012 to Richardson, U.S. Pat. No. 4,132,578 to Gell, the '576 patent issued to van Boekel, U.S. Pat. No. 5,360,333 to Schmidt, and U.S. Pat. No. 5,558,888 to Beck. The thermal clamping force concepts disclosed by these patents largely depend on the temperature generated by the heater and to the difference in response of the materials exhibiting thermal expansion coefficients to the cycling temperature of the molding process.
It has been noticed that the "thermal" clamping methods using bi-metallic elements work properly only within a limited "temperature operating window". For example, the clamping design disclosed in the '333 patent to Schmidt, assigned to the assignee of the instant application, provides a good clamping force at temperatures below 500.degree. F. This limitation is caused amongst others by factors that depend on the thermal behavior of the bi-metallic elements. In some cases, they may reach a thermal expansion plateau above certain temperatures. In other cases, their thermal expansion range may be diminished after a large number of molding cycles where the temperature goes from the cold to the hot conditions during the actual operation.
The current invention teaches a novel clamping method and system that overcome the problems of the known systems by providing a "flexible" and "thermal" clamping means to form a removable hybrid clamping system that operates efficiently at temperatures below and over 500.degree. F.
More specifically, the current invention discloses a new "flexible" clamping means that generates sufficient clamping force in both cold and warm conditions, whether or not the thermal expansion of the elements forming the heater system contribute to the development of an additional clamping force in conjunction with the "flexible" clamping means.